There is an increasing need for biofuels, suitable as liquid fuels as such, particularly transportation fuels or compatible with said fuels. Biofuels are typically manufactured from feedstocks originating from renewable sources including oils and fats from plants, animals, algae, fish and various waste streams and sewage sludge. The common feature in these feedstocks is that they are composed of glycerides and free fatty acids, both of these containing aliphatic carbon chains having from about 8 to about 24 carbon atoms and the aliphatic carbon chains being saturated, or mono-, di- or polyunsaturated. Catalytic hydroprocessing of these materials requires large quantities of hydrogen, and this is a major operating cost in the production of biomass-derived fuels by catalytic hydroprocessing. Further, it is generally more difficult to convert low quality feedstocks of more heterogeneous nature and containing contaminants by catalytic hydroprocessing, or more complicated equipment is required.
Recycling of excess hydrogen to hydroprocessing is commonly used in hydroprocessing. Hydroprocessing of heterogeneous feedstocks originating from renewable sources produces light hydrocarbons as unwanted byproducts. Light hydrocarbons are separated in the course of the process from the process liquid in gas separation, where hydrogen is separated and recycled to the hydroprocessing reactor.
Typically, in a continuously operating process light hydrocarbons, particularly methane is concentrated in the hydrogen recycle stream, which results in the reduction of hydrogen partial pressure in said stream and, further, via that reduction also the hydrogen partial pressure in the hydroprocessing reactor(s) is reduced. For achieving required product properties, such as specific diesel grade, significant amounts of hydrogen make-up gas are necessary for maintaining required hydrogen partial pressure. As methane does not significantly react at hydroprocessing temperatures, it will build up in hydrogen recycle streams. Thus high hydrogen purge to flare is required for producing high value products.
Hydrogen is typically supplied to hydroprocessing processes from a hydrogen plant operating most commonly by steam reforming. In hydrogen plants, in the steam reforming process (typically SMR=steam methane reforming) usually natural gas, liquefied petroleum gas (LPG) gas or naphtha is used as starting material. Methane (and other light hydrocarbons) in the starting material react at elevated temperatures in the presence of a nickel based catalyst with steam to yield synthesis gas containing carbon monoxide and hydrogen, followed by water gas shift reaction at a lower temperature, where said carbon monoxide reacts with water to produce carbon dioxide and hydrogen. The catalysts in the hydrogen plant do not tolerate any sulfur and thus sulfur removal reactors are arranged upstream from the reformer. In said sulfur removal (desulfurization) reactors high molecular weight sulfur compounds are hydrogenated to hydrogen sulfide, followed by treating with absorption beds for removing the hydrogen sulfide. This requires hydrogen, which is typically obtained from the hydrogen plant and consumes thus hydrogen.
The control and adjusting of hydrogen production capacity from the hydrogen plant is very slow because of the high temperature nature of said plant. This means that in practice the hydrogen plant is operated to continuously provide an excess of hydrogen and the hydrogen which is not consumed is directed to flare and wasted.
Despite the ongoing research and development of processes for the manufacture of liquid fuels, there is still a need to provide an improved process for producing hydrocarbons useful as liquid fuels or fuel blending components.